Staphylococcus aureus (S. aureus) is a bacterium that commensally colonizes more than 25% of the human population. Upon gaining access to the bloodstream, S. aureus disseminates resulting in a wide range of diseases. S. aureus is the leading cause of nosocomial infections, is the most common etiological agent of infectious endocarditis as well as skin and soft tissue infections, and is one of the four leading causes of food-borne illness. Altogether, S. aureus is estimated to infect more than 1.2 million patients per year in USA hospitals. The threat of S. aureus to human health is further highlighted by the emergence of antibiotic-resistant strains (i.e., MRSA strains), including strains that are resistant to vancomycin, an antibiotic considered the last line of defense against S. aureus infection. These facts highlight the importance of developing novel therapeutics against this important pathogen.
The success of S. aureus as a human pathogen is in part due to the ability of this bacterium to disarm the host's immune system by producing an arsenal of virulence factors that are secreted into the extracellular milieu (Foster, T. J. “Immune Evasion by Staphylococci,” Nat. Rev. Microbiol. 3:948-58 (2005)). Among these, the bicomponent, pore-forming leukotoxins are of particular interest because they target and kill a variety of immune cells involved in infection-control (Vandenesch et al., “Staphylococcus aureus Hemolysins, Bi-Component Leukocidins, and Cytolytic Peptides: A Redundant Arsenal of Membrane-Damaging Virulence Factors?” Front. Cell. Infect. Microbiol. 2:12 (2012); Alonzo & Torres, “Bacterial Survival Amidst an Immune Onslaught: The Contribution of the Staphylococcus aureus Leukotoxins. PLoS Pathog. 9:e1003143 (2013)). Among these immune cells, leukotoxins kill polymorphonuclear cells also known as “PMNs”, which act as the initial barrier to infection by means of phagocytic killing of the intruding microorganism (Rigby & DeLeo, “Neutrophils in Innate Host Defense Against Staphylococcus aureus Infections,” Semin. Immunopathol. 34:237-59 (2012)).
Each leukotoxin is composed of two subunits, the “S” and “F” type, which act in concert to form octameric pores in target cell membranes (Yamashita et al., “Crystal Structure of the Octameric Pore of Staphylococcal Gamma-Hemolysin Reveals the Beta-Barrel Pore Formation Mechanism by Two Components,” Proc. Nat'l. Acad. Sci. U.S.A. 108:17314-9 (2011)), ultimately leading to cell death. Clinically relevant strains of S. aureus can produce up to five different bicomponent leukotoxins: Panton-Valentine leukocidin (PVL or LukFS-PV), leukocidin E/D (LukED), γ-hemolysin (HlgAB and HlgCB), and leukocidin A/B (LukAB; also known as LukGH) (Vandenesch et al., “Staphylococcus aureus Hemolysins, Bi-Component Leukocidins, and Cytolytic Peptides: A Redundant Arsenal of Membrane-Damaging Virulence Factors?” Front. Cell. Infect. Microbiol. 2:12 (2012); Alonzo & Torres, “Bacterial Survival Amidst an Immune Onslaught: The Contribution of the Staphylococcus aureus Leukotoxins. PLoS Pathog. 9:e1003143 (2013)). These toxins are each capable of targeting and killing human PMN, but they also exhibit tropism towards additional leukocytes (Alonzo & Torres, “Bacterial Survival Amidst an Immune Onslaught: The Contribution of the Staphylococcus aureus Leukotoxins. PLoS Pathog. 9:e1003143 (2013); Gravet et al., “Characterization of a Novel Structural Member, LukE-LukD, of the Bi-Component Staphylococcal Leucotoxins Family,” FEBS Lett. 436:202-8 (1998); Morinaga et al., “Purification, Cloning and Characterization of Variant LukE-LukD With Strong Leukocidal Activity of Staphylococcal Bi-Component Leukotoxin Family,” Microbiol. Immunol. 47:81-90 (2003); Perret et al., “Cross-Talk Between Staphylococcus aureus Leukocidins-Intoxicated Macrophages and Lung Epithelial Cells Triggers Chemokine Secretion in an Inflammasome-Dependent Manner,” Cell. Microbiol. 14:1019-36 (2012)), suggesting that S. aureus uses leukotoxins to deplete the immune cells responsible for protecting the body from infection. In addition to leukocytes, HlgAB, HlgCB and LukED can also lyse red blood cells (RBC) (Morinaga et al., “Purification, Cloning and Characterization of Variant LukE-LukD With Strong Leukocidal Activity of Staphylococcal Bi-Component Leukotoxin Family,” Microbiol. Immunol. 47:81-90 (2003)), which could contribute to S. aureus growth in vivo by releasing hemoglobin from RBC for use as an iron source (Torres et al., “Staphylococcus aureus Fur Regulates the Expression of Virulence Factors That Contribute to the Pathogenesis of Pneumonia,” Infect. Immun. 78:1618-28 (2010)).
Despite more than one hundred years of investigation into the cytotoxic activity of S. aureus leukotoxins, the cellular receptors that dictate the tropism of leukotoxins to immune cells and RBC remain incompletely defined.
The present invention is directed to overcoming these and other limitations in the art.